1. Field of the Invention
Embodiments of the present invention generally relate to inline continuous formation of photovoltaic solar cells. In particular, embodiments of the invention relate to improving the selenization process for forming copper indium gallium selenium type photovoltaic solar cells.
2. Description of the Related Art
Solar cells are photovoltaic (PV) devices that convert sunlight directly into electrical power. PV devices typically have one or more p-n junctions, sometimes referred to as a homojunction p-n system. Each junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region. When the p-n junction of the PV cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through the PV effect. PV solar cells generate a specific amount of electric power, and cells are tiled into modules sized to deliver the desired amount of system power. PV modules are joined into panels with specific frames and connectors. The solar cells are commonly formed on a silicon substrate, which may be in the form of single or multicrystalline silicon substrates or have microcrystalline or amorphous type silicon films deposited on substrates. A typical PV cell includes a p-type silicon substrate or sheet typically less than about 0.3 mm thick with a thin layer of n-type silicon on top of a p-type region formed in a substrate.
Other types of solar cells may use a more complex heterojunction system to generate electricity. A heterojunction is an interface that occurs between two layers of dissimilar crystalline semiconductors creating unequal band gaps as opposed to a homojunction. One example of a heterojunction system is a copper indium gallium selenide type solar cell or CIGS.
The photovoltaic market has experienced growth with annual growth rates exceeding above 30% for the last ten years. Some articles have suggested that solar cell power production world wide may exceed 10 GWp in the near future. It has been estimated that more than 95% of all photovoltaic modules are silicon wafer based. The high market growth rate in combination with the need to substantially reduce solar electricity costs has resulted in a number of serious challenges for inexpensively forming high quality photovoltaic devices. Therefore, one major component in making commercially viable solar cells is reducing the manufacturing costs required to form the solar cells, such as by improving the device yield and increasing the substrate throughput.
One of the biggest hindrances to reducing current manufacturing costs of CIGS type solar cells is the inability to efficiently implement the selenization operation in an inline continuous production process. Current processes are not well adapted for inline continuous production and also may lack CIGS phase uniformity. Therefore, there is a need in CIGS type solar cell manufacturing to provide apparatuses and processes that adequately implement the selenization process of CIGS type solar cells to improve the device yield, CIGS film uniformity, and produce a lower cost of ownership (CoO) than other known apparatuses and methods.